a) Field of the Invention
The present invention relates to an objective lens system for microscopes which comprises a graded refractive index lens or such lenses having refractive index distribution in the direction perpendicular to the optical axis (radial GRIN lenses).
b) Description of the Prior Art
It is generally difficult in objective lens systems for microscopes to correct astigmatism and curvature of field with a small number of lens elements. When chromatic aberration is to be corrected additionally, refractive indices and Abbe's numbers of the glass materials to be used must be taken into consideration and therefore impose restrictions on the correction of the aberrations, thereby making it more difficult to correct the aberrations.
In order to realize an objective lens system for microscopes which satisfies the requirements such as working distance, numerical aperture, magnification etc., while overcoming the problems mentioned above, it was conventionally required to increase the number of lens elements composing the objective lens system.
In order to compose objective lens systems for microscopes wherein aberrations are favorably corrected with small numbers of lens elements, graded refractive index lenses (GRIN lenses) are used.
As the objective lens systems for microscopes using GRIN lenses, there are known the lens systems disclosed by Japanese Patents Kokai Publication No. Sho 62-31816, No. Sho 62-31817 and No. Sho 62-34117, etc. Further, as the publications containing descriptions on optical systems other than objective lens systems for microscopes and correction of aberrations, there are known Japanese Patents Kokai Publication No. Sho 60-159817, No. Sho 61-4012 and No. Sho 62-153909.
Out of these conventional examples, the lens systems disclosed by Japanese Patents Kokai Publication No. Sho 60-159817, No. Sho 61-5222 and No. Sho 62-153909 are photographic lens systems having corrected axial aberrations, offaxial aberrations, and chromatic aberration of the d-line and g-line. However, these publications contain no description on values of the terms of high orders N.sub.1 and N.sub.2 of the refractive index distribution coefficient of the GRIN lenses for the C-line and F-line in the numerical data related to chromatic aberration. Further, since these publications take no consideration on practical manufacturing circumstances of the glass materials, the GRIN lenses described therein have very low possibilities to be manufactured in practice.
Furthermore, the inventions disclosed by Japanese Patents Kokai Publication No. Sho 60-163015 and No. Sho 63-124011 have relations to GRIN lenses, but contains no descriptions on the offaxial aberrations, especially curvature of field.
Japanese Patent Kokai Publication No. Sho 62-34117 discloses an objective lens system for microscopes using GRIN lenses but takes no consideration on correction of chromatic aberration.
Japanese Patents Kokai Publication No. Sho 62-31816 and No. Sho 62-31817 disclose objective lens systems for microscopes which are of the type, similar to that of the objective lens system according to the present invention, and wherein not only the axial aberrations and offaxial aberrations but also chromatic aberrations of the C-line and F-line are corrected. For the GRIN lenses used in these objective lens systems, however, the characteristics of the glass materials currently available are not considered. Speaking concretely, the values of v.sub.1.sbsb.d determined from the distribution coefficient of the second order of the GRIN lenses are smaller than 0. Accordingly, these GRIN lenses have low possibilities to be manufactured in practice. The objective lens system disclosed by Japanese Patent Kokai Publication No. Sho 62-31816, for example, uses a GRIN lens in the first lens unit and selects v.sub.1.sbsb.d &lt;0 for correction of chromatic aberration. Taking the characteristics of the glass materials currently available into consideration, the GRIN lens will have a low possibility to be manufactured unless v.sub.1.sbsb.d is larger than 0. v.sub.1.sbsb.d larger than 0 will make it impossible to correct chromatic aberration in this conventional example.